A mobile telephone network, for example a UMTS (Universal Mobile Telecommunications system) network, conventionally consists of a wireless access network, comprising a plurality of public access stations, known as base stations, and a core network that handles management of the service offered and routing of calls to fixed networks such as the public switched telephone network, the Internet, etc. Such mobile telephone networks are generally organized in cells, each associated with a base station, and the size of which varies as a function of the user density, the geography of the terrain, the power of the associated base station, etc. In a GSM network, for example, the adjacent cells of the network use different carrier frequencies.
Each mobile telephone network is managed by an operator, is associated with a geographical area (conventionally a state), and is identified by a PLMN (Public Land Mobile Network) code that is specific to it.
A mobile terminal can access only the mobile telephone network managed by the operator of its subscription or possibly another network with which its operator has entered into reciprocal roaming agreements. After being connected to a PLMN for which it is authorized, the mobile terminal monitors signals coming from the various access stations that it receives, selects the signal received with the best quality, and attaches to the cell from which it comes.
The terminal effects this monitoring on a quasi-permanent basis, with the result that, as soon as it detects a signal coming from a new base station, it measures certain parameters of the signal received in order to determine whether it is of better quality than the signal coming from the cell to which it is currently connected. If so, and if the terminal is in idle mode, it can quit its current cell and attach to the cell from which it is receiving a signal of better quality.
If the terminal is in connected mode (i.e. if a call is in progress), the network requests it to send back measurements on the adjacent cells. The network can then instruct the mobile terminal to change cell. In practice, this handover occurs mainly when the mobile terminal is moving around, causing it to enter or leave a cell whose size is normally in the range 100 meters (m) to 30 kilometers (km) in a GSM or UMTS network, for example.
The current expansion of mobile communications networks is now oriented to offering convergence between fixed telephone networks (such as the public switched telephone network (PSTN) or the Internet) and mobile networks.
In this context, some operators are envisaging offering their customers, whether private persons or businesses, second generation (2G), third generation (3G), or beyond third generation (B3G) wireless coverage in the home or on business premises, for example in the form of a private wireless access station (in the home or on business premises), connected to an ADSL (Asynchronous Digital Subscriber Line) modem or to any other equipment providing access to a high-bit-rate network (for example of the FTTH (Fiber To The Home) type). Below, for reasons of simplification, the term private wireless access station designates the combination of the access station proper and the high-bit-rate network access equipment to which it is connected.
Each person can therefore have their own home wireless access station, associated with a list of users authorized to access it (family members, friends, etc.). An access control mechanism could bar access to this private station to users not included in the list.
Similarly, in the business context, a business could have one or more wireless access stations on its site, access to which would be limited to the employees of the business and prohibited to visitors, for example.
However, introducing this kind of home wireless coverage service encounters the problem of increasing the number of access stations to the communications network of the operator, a large number of such access stations being likely to be added to the base stations (public access stations) already deployed by the operator.
A mobile terminal moving around in the communications network of the operator would frequently detect new signals coming from public or private access stations near it and to which it could thus be tempted to attach.
Thus a mobile terminal moving around in a built-up area or in a building in which all homes are equipped with private access stations to the service would tend to be tempted to attach to each of those stations in succession, as soon as it detected that the signal from one of them is of sufficient level.
Now, throughout the stage in which a mobile terminal attempts to attach to a wireless access station, is rejected if access is refused, and then attempts to attach again, it cannot be contacted from the communications network; during this period, the duration of which is normally of the order of one second, the mobile terminal ceases to monitor the signals being transmitted in the area in which it is marked as being connected in the network.
If these unsuccessful attempts to attach to private or base stations were to be repeated too frequently, the user of the mobile terminal could miss incoming calls, which is a particular nuisance.
This proves particularly problematic for users of mobile terminals who do not subscribe to the new home wireless coverage service. Introducing a large number of private access stations which they might not be authorized to access because they lack a subscription, would interfere with the operation of their mobile terminals and degrade their performance by leading to multiple attempts to attach the terminals to these private access stations.
There is therefore a need for a technique that enables a communications network operator to offer a new home wireless coverage service to its subscribers, without the new service interfering with users of the network who are not themselves subscribers.